Conventionally casing treatment slots are provided on the inner surface of a compressor casing, or a fan casing, around the tips of the compressor blades, or fan blades, to extend the stable flow range over which the compressor, or fan may operate.
Typically casing treatment slots are provided around the first stage of compressor blades or around the fan blades.
Our UK patent GB1518293 discloses a compressor casing treatment comprising a plurality of circumferentially spaced slots in the inner surface of the compressor casing and around a stage of compressor blades. The slots are arranged at an angle to the axis of rotation of the compressor blades.
Our UK patent GB2245312B discloses a compressor casing treatment comprising a plurality of circumferentially spaced slots in the inner surface of the compressor casing and around a stage of compressor blades. The slots are arranged at an angle to the axis of rotation of the compressor blades. The slots are also arranged at an angle to the radial direction. The upstream edges of the slots are upstream of the leading edges of the compressor blades and the trailing edges of the slots are upstream of the trailing edges of the slots.
In each of these arrangements the slots are identical, they have the same length, the same depth, the same width, the same angle of inclination to the axis of rotation and the same angle of inclination to the radial direction etc.
However, the compressor casing suffers from cracking of the webs, the pieces of compressor casing circumferentially between the compressor casing treatment slots. It is believed that the cracking of the webs occurs due to the unsteady pressure acting on them due to the periodic passing of the compressor blades. All the slots are identical and have the same geometry and thus they experience the same unsteady pressure variations but with a time lag related to the passing frequency of the compressor blades. In adjacent slots there will be a time lag between the pressure variations and thus the pressure variations are out of phase and this results in a pressure difference across the webs. Additionally because all the compressor blades are substantially identical and all the casing treatment slots are substantially identical, the presence of an incoming distortion may substantially increase the unsteady forces on the rotor creating further phase differences within the casing treatment slots and thence unsteady forces on the webs.